One of the important technical subjects in full color copying machines is to enlarge the dynamic range for copying images in order to achieve the overall fidelity of color reproduction. Explained using a .gamma. characteristic curve showing the relation between an original document density and a copied image density, for example, as shown by a curve L in FIG. 15, the dynamic range is given by a maximum value of the original document density within a range wherein the curve L has a sloped portion, and is used as a measure for indicating tone gradations. In normal cases the dynamic range is as small as 0.6, far from an optimal value of 2.0, and the value is not satisfactory in practical use in terms of reproducibility of colors.
For that reason, attempts have been made to enlarge the dynamic range by the use of the simplest methods, such as adjustment of an exposure light amount or adjustment of a surface voltage of a photoreceptor. However, those methods have a problem that a copied image tends to get entirely faded due to lowering of saturated density.
In order to solve the problem, conventionally, those methods such as superposition method and screen method have been formulated and come into practical use. In the superposition method, as is shown in FIG. 15, it is arranged that the characteristic curve L for a copied image may be brought near an optimal curve shown by an alternate long and two short dashes line in FIG. 15 (where an original document density coincides with a copied image density), by superposing an image with low density components which has characteristics indicated by a curve G and an image with high density components which has characteristics indicated by a curve H. The superposition method provides copied images having, for example, a large dynamic range of substantial 1.0 to 1.2, and also permits a higher saturated density, thereby improving tone gradations, especially in high density portions.
On the other hand, in the screen method, as is illustrated in FIG. 16(a), a screen filter 43 which is installed at a vicinity of a photoreceptor 42 charged by a main charger 41, is adapted to filter light for exposure reflected from an original document into a pattern of lines, and as is illustrated in FIG. 16(b), tone gradations which are dependent upon a width of the lines of the screen filter 43 can be provided to a copied image. By using the screen method, a .gamma. characteristic is shifted as is indicated by a curve J in FIG. 17, and brought closer to the optimal curve in its low density portion compared with a curve K indicating a normal .gamma. characteristic, thereby improving tone gradations in its low density portion. Consequently, a comparatively large dynamic range of substantial 0.8 to 1.0 can be obtained.
In the superposition method, although the saturated density of copied images is increased in their high density portion, no treatment is applied to improve tone gradations in their low density portion. Consequently, since the method can not provide good tone gradations within original document image densities of substantial 0.1 to 0.5, it fails to clearly reproduce low density portions to be used for portraying human faces or other objects.
Moreover, in the screen method, since an exposure is performed through a screen filter on an entire area having different densities, an amount of exposure light is extremely attenuated. Accordingly, it is difficult to apply screen filters in practical use, especially, to full color copying machines wherein light is subjected to color separation before exposure. Further, in the case where an original document image is composed of characters, lines, or the like as is illustrated in FIG. 18(a), the screen method has a problem that a copied image obtained by exposure through a filter presents a dot-like appearance as illustrated in FIG. 18(b), and characters thereon are not clear and difficult to read.
In order to solve the above problems, a color copying machine disclosed by Japanese Patent Laid-Open Publication No. 206565/1987 (Tokukaisho 62-206565) is arranged to employ both of the superposition and screen methods. The color copying machine is provided with two exposing devices, and for the same original document image, one of the exposing devices having a larger amount of exposure light conducts a direct exposure, while the other exposing device having a smaller amount of exposure light conducts an exposure through a filter. Then the resulting electrostatic latent images are superposed on a photoreceptor.
The above color copying machine permits high tone gradations at a high density portion of a copied image by means of the direct exposure, while improving tone gradations at a low density portion thereof by the use of the screen filter. Moreover, the screen filter is not used for the high density portion where a larger amount of exposure light is required compared with the low density portion, and therefore it is not necessary to increase the amount of exposure light.
Furthermore, for an image containing many characters and lines, the exposure by the use of the screen filter is not performed, thereby improving sharpness of the image.
However, in the conventional color copying machine wherein electrostatic latent images are superposed on the photoreceptor a plurality of times, when the photoreceptor whereon an electrostatic latent image has been formed is uniformly charged, characters or lines contained in an original document image may become thinner due to the effect of the electrostatic latent image already formed. Further, the color copying machine should be provided with a plurality of exposure devices, thereby causing the number of parts to increase and the cost to become high.